A color picture tube includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of a faceplate of the tube and is made up of an array of elements of three different color emitting phosphors. An apertured mask, called a shadow mask, is interposed between the gun and the screen to permit each electron beam to strike only the phosphor elements associated with that beam.
A shadow mask is a thin sheet of metal, such as steel, that is contoured to somewhat parallel the inner surface of the tube faceplate. The shadow mask includes a large central apertured portion, a solid border portion surrounding the apertured portion and a peripheral skirt portion. The skirt portion is angled from the other portions of the mask and is usually welded to a peripheral frame that supports the mask within a tube.
In making a shadow mask, a flat sheet of metal is etched to form the apertures, which are usually elongated slots or circular holes. Thereafter, the sheet is formed into the desired contour, such as spherical or biradial, and a skirt is formed by sweeping back the peripheral edge of the sheet. During mask forming, it is sometimes desirable to include a peripheral U-shaped bead in the border area of the mask to provide reinforcement of the mask, particularly to prevent or minimize mask buckling during tube operation, when the masks expands.
In a prior conventional method of forming a shadow mask, a flat apertured mask first is clamped around its peripheral edges and a die presses the mask against a pad to form the mask contour. A bead is formed in the mask by including a peripheral groove and protrusion on the punch and pad, respectively, to form the bead at the same time the mask contour is formed. One problem with this prior method is that, during forming of the bead, material is stretched on both sides of the bead. This stretching often causes the mask apertures at the edge of the apertured portion to also stretch. Such stretching of the apertures may rupture tie bars between apertures or may stretch the sizes of the apertures beyond design limits. Therefore, there is a need for a method of forming a shadow mask with a bead that will not cause a stretching of the apertured portion because of the addition of the bead to the mask. The present invention accomplishes this result by a method that forms a bead in a separate step that does not stretch the apertured portion of a shadow mask.